Why Isn’T My Kitten More Energetic?

what can i do to help her?

I have a 5 month old kitten that has never been on antibiotics. She is purring all the time and very happy, but she is also sleepy most of the day. I took her off one med due to side effects since it was making her lethargic. Now I am concerned about how much she eats, as she still consumes less than half of what my other cats eat. What can I feed this cat now? How often should I change her litter box, etc.? Any advice would be helpful! Thank you for your time!!